14,508 research outputs found
Peaceman-Rachford splitting for a class of nonconvex optimization problems
We study the applicability of the Peaceman-Rachford (PR) splitting method for
solving nonconvex optimization problems. When applied to minimizing the sum of
a strongly convex Lipschitz differentiable function and a proper closed
function, we show that if the strongly convex function has a large enough
strong convexity modulus and the step-size parameter is chosen below a
threshold that is computable, then any cluster point of the sequence generated,
if exists, will give a stationary point of the optimization problem. We also
give sufficient conditions guaranteeing boundedness of the sequence generated.
We then discuss one way to split the objective so that the proposed method can
be suitably applied to solving optimization problems with a coercive objective
that is the sum of a (not necessarily strongly) convex Lipschitz differentiable
function and a proper closed function; this setting covers a large class of
nonconvex feasibility problems and constrained least squares problems. Finally,
we illustrate the proposed algorithm numerically
Self-cancellation of ephemeral regions in the quiet Sun
With the observations from the Helioseismic and Magnetic Imager aboard the
Solar Dynamics Observatory, we statistically investigate the ephemeral regions
(ERs) in the quiet Sun. We find that there are two types of ERs: normal ERs
(NERs) and self-cancelled ERs (SERs). Each NER emerges and grows with
separation of its opposite polarity patches which will cancel or coalesce with
other surrounding magnetic flux. Each SER also emerges and grows and its
dipolar patches separate at first, but a part of magnetic flux of the SER will
move together and cancel gradually, which is described with the term
"self-cancellation" by us. We identify 2988 ERs among which there are 190 SERs,
about 6.4% of the ERs. The mean value of self-cancellation fraction of SERs is
62.5%, and the total self-cancelled flux of SERs is 9.8% of the total ER flux.
Our results also reveal that the higher the ER magnetic flux is, (i) the easier
the performance of ER self-cancellation is, (ii) the smaller the
self-cancellation fraction is, and (iii) the more the self-cancelled flux is.
We think that the self-cancellation of SERs is caused by the submergence of
magnetic loops connecting the dipolar patches, without magnetic energy release.Comment: 6 pages, 4 figures, accepted for publication in ApJ
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